An apple may not fall far from the tree, but for a leaf, it depends on its shape.
Elm or apple leaves — oval and symmetrical, with few protruding lobes — fall quickly, making them likely to end up close to the tree’s base. Adding lobes and asymmetry to leaves slows their descent, causing them to fall farther away, physicists Matthew Biviano and Kaare Jensen report May 7 in Journal of the Royal Society Interface.
Deciduous trees shed their leaves every year — bidding about 40 percent of their built-up carbon and nutrients goodbye. But it doesn’t all have to be lost. If leaves drop near a tree’s base, it has access to those nutrients again when they decompose. And where the leaves land is determined by the wind, the weather and their shape. Seeds need to spread, so a plant may want those carried by the wind, says Andreas Carlson, a physicist at the University of Oslo who was not involved in the study. Leaves, on the other hand “need to really settle fast to provide nutrients.”
Jensen and Biviano, both at the Technical University of Denmark in Kongens Lyngby, analyzed the falling leaves of 25 different tree species — including some within the families of oaks, elms, maples and apples. They also included the common laboratory herb Arabidopsis, which has a mutated gene that produces asymmetrical leaves. The scientists laser cut paper replicas of three leaves from each species, and dropped them into a tank full of water. The water slowed the leaves descent, allowing the scientists to analyze the relationship between leaf shape and fall speed. Most leaves fell quickly, — even lobed oak leaves — within 10 percent of the speed of the fastest possible shape (a circle, which fell in their tank at 1.37 centimeters per second), while the mutated, asymmetrical leaves of Arabidopsis fell 15 percent slower.
The scientists were able to show that the most important variables in how quickly a leaf fell were the leaf’s symmetry and its “lobedness,” or how many lobes it has. Then they used these variables to add “mutations” to their paper leaves, bumping up their asymmetry and lobes. The resulting leaves fell 15 to 30 percent slower. Asymmetry was most important to falling speed and resulted in leaves that whirled slowly as they fell. “Symmetry is more important than lobes, as simple as that,” Jensen says. “However, the slowest shape we observed (not the slowest possible) was both asymmetrical and had lobes. Therefore, there is some interplay between these two factors.”
Leaf shape is probably also determined by a tree’s climate, nutrient access, water availability and the work leaves do to exchange carbon dioxide for oxygen. “We are not saying that settling aerodynamics is the only driver of symmetry,” Jensen says. “It is simply a nice and simple factor.”
Jensen and Biviano concluded that the natural symmetry and few lobes of natural leaves make it more likely that trees can get access to their carbon as they decay. “It really nicely highlights … how their shapes might be somewhat useful in terms of also nutrient recycling,” Carlson says. With all the nutrients trees can keep by dropping leaves close to home, it might not be easy to make like a tree and leaf.